Flexible joints having an annular elastomeric flex element mounting an extension to a housing are used for reducing motion-induced stresses between floating offshore facilities and risers and tendons depending from the floating offshore facilities. Typically the flex element consists of alternating spherical shims of metal, or other stiff material, and layers of elastomeric material. Such a flex element is capable of providing a free angular displacement of about ±15 degrees or more while supporting an axial tension proportional to the size of the flex element. Typically the size of the flex element has been selected to handle the desired load upon the riser or tendon, and flex elements have been manufactured and stocked in various sizes for handling various standard sizes of risers or tendons.
Risers are used for transfer of production fluids from the sea floor up to a deck of a floating offshore vessel, and for transfer of the production fluid off the vessel to one or more export lines. The loads impressed by the riser on a flex element typically consist of tension in the riser, angular displacement and rotation of the riser, internal pressure in the production fluid, and increased temperature from the production fluid. Thus, the internal pressure in the production fluid, and increased temperature from the production fluid, may make the selection of a flex element for a riser more difficult than the selection of a flex element for a tendon.
For various applications, flexible pipe joints have incorporated more than one flex element in a common housing. For example, a double-ended flexible pipe joint for a riser has a first flex element in the housing for mounting a first extension pipe to the housing, and a second flex element in the housing for mounting a second extension pipe to the housing. The two extension pipes extend in opposite directions from the common housing. In this fashion, the double-ended flexible pipe joint can accommodate twice the angular displacement than can be tolerated by a single-ended flexible pipe joint having a single flex element. The angular displacement is divided between the two flex elements in the double-ended flexible pipe joint, but each of the two flex elements carries the same full tension of the riser. Examples of such double-ended flexible pipe joints are found in Herbert et al. U.S. Pat. No. 3,680,895 issued Aug. 1, 1972; Herbert et al. U.S. Pat. No. 4,068,864 issued Jan. 17, 1978 (see FIG. 4); and Whightsil, Sr. et al. U.S. Pat. No. 5,133,578 issued Jul. 28, 1992.
Flexible pipe joints have incorporated more than one flex element in a common housing so that two flex elements are subjected to the same angular displacement yet only one of these two flex elements carries the tensile load upon the flexible pipe joint. Such an arrangement may reduce pressure from production fluid on each flex element and provide both a primary and a backup sealing mechanism for containing the pressurized production fluid within the pipe joint. However, the flex elements in these concepts need to be pre-compressed for proper functioning; a fact that reduces the usable life of the flex elements. Thus, these designs make an inefficient use of the two flex elements both to carry the axial load upon the pipe and to seal the pressure. Examples of such flexible pipe joints are found in Schwemmer U.S. Pat. No. 4,183,556 issued Jan. 15, 1980; Ohrt U.S. Pat. No. 4,068,868 issued Jan. 17, 1978; Peppel et al. U.S. Pat. No. 4,784,410 issued Nov. 15, 1988; and Peppel et al. U.S. Pat. No. 4,984,827 issued Jan. 15, 1991.
A pipe joint having two flex elements in a common housing and different levels of axial pre-compression upon the two flex elements is disclosed in Lamy et al. U.S. Pat. No. 4,416,473 issued Nov. 23, 1983. The two flex elements are arranged on opposite sides of a common center of rotation. The pipe joint has a flange and a collar forming a spherical bearing permitting angular displacement but preventing relative movement under axial compression. (Lamy, col. 5, lines 2-8.) One flex element having a larger diameter takes up axial tensile loads. The other flex element having a smaller diameter is designed to ensure solely the sealing of the fluid within the pipe. (Lamy, col. 5 lines 16-34.) A desired axial pre-stress is applied to the flex element having the smaller diameter by pulling bolts up tighter to close a clearance of a cylindrical opening. (Lamy, col. 6, lines 30-46.)